#include<bits/stdc++.h>
using namespace std;
typedef pair<int, int> PII;

const int N = 1e5 + 10;

int n, m;
int h[N], e[N], w[N], ne[N], idx;
int dist[N];
bool st[N];

void add(int a, int b, int c)  // 添加一条边a->b，边权为c
{
    e[idx] = b, w[idx] = c, ne[idx] = h[a], h[a] = idx ++ ;
}


int spfa()  // 求1号点到n号点的最短路距离，如果从1号点无法走到n号点则返回-1
{
    memset(dist, 0x3f, sizeof dist);
    dist[1] = 0;
    
    queue<int> q;
    q.push(1);
    st[1] = true;
    
    while (q.size()) {
        int t = q.front(); q.pop();
        
        st[t] = false;
        
        for (int i = h[t]; i != -1; i = ne[i]) {
            int j = e[i];
            if (dist[j] > dist[t] + w[i]) {
                dist[j] = dist[t] + w[i];
                if (!st[j]) {
                    q.push(j);
                    st[j] = true;
                }
            } 
        }
    }
    
    if (dist[n] > 0x3f3f3f3f / 2) return -1;
    return dist[n];
    
}

int main()
{
    scanf("%d%d", &n, &m);
    
    memset(h, -1, sizeof h);
    
    while (m -- ) {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
    }

    int t = spfa();
    
    if (t == -1) puts("impossible");
    else printf("%d", t);

    return 0;
}